what is differece between clock and bus speed

Is there any difference between Clock speed and Bus Speed.I am taking reference from a computer article which i read on webpage.
""Clock speed - Intel manufactures the Pentium 4 chips to run at a higher clock speed than Celeron chips. The fastest Pentium 4 might be 60 percent faster than the fastest Celeron.

Bus speed - There are differences in the maximum bus speeds that the processors allow. Pentium 4s tend to be about 30 percent faster than Celerons. ""
Kindly explain me difference between two.
And secondly what is difference between Celeron 1.1GHz and Celeron 1.1AGHz.
For what does "A" satnds for.AS Far as know 1.1AGHz is .18micron and 1.1AGHz is .13micron Technology.And cache of second one is 256K while of first one is 128K.Any other difference plz tell me.:angel: